The present invention relates generally to photomasks used for producing high-density integrated circuits such as LSIs and VLSIs and more particularly to phase shift layer-containing phase shift photomasks designed for forming fine patterns with high accuracy.
So far, semiconductor integrated circuits such as ICs, LSIs and VLSIs have been produced by repeating a so-called lithographic process wherein resists are applied on the substrates to be processed, like Si wafers, and the substrates are then exposed to light through steppers (step-and-repeat photolithographic systems with demagnification) or other like hardware to form the desired patterns, followed by development and etching.
The photomasks used in such a lithographic process and called "reticles" are now increasingly required to have much higher accuracy as current semiconductor integrated circuits are higher in performance and integration than ever before. Referring to a DRAM that is a typical LSI as an example, a dimensional variation of a five-fold reticle for a 1 megabit DRAM, i.e., of a reticle five-fold greater in size than the pattern to be exposed to light, is required to have an accuracy of 0.15 .mu.m even where the mean value=.+-.3.sigma. (.sigma. is standard deviation). Likewise, five-fold reticles for 4- and 16-megabit DRAMs are required to have an accuracy of 0.1 to 0.15 .mu.m and 0.05 to 0.1 .mu.m, respectively.
Furthermore, the line widths of device patterns formed with these reticles must be much finer, say, 1.2 .mu.m for 1-megabit DRAMs and 0.8 .mu.m for 16-megabit DRAMs, and various photolithographic processes are-now being studied to meet such demand.
With the next, . . . generation device patterns of, e.g., the 64-megabit DRAM class, however, "stepper" systems using conventional reticles have been found to place some limitation on the resolution of resist patterns. Thus, a version of reticle based on a new idea, like those set forth in JP-A-58-173744 laid open for public inspection, JP-B-62-59296, etc., and referred to as phase shift masks, has been proposed in the art. Phase shift lithography using this reticle is a technique designed to improve the resolving power and contrast of a projected image by controlling the phase of light transmitting through the reticle.
Phase shift lithography will now be briefly explained with reference to FIGS. 4a-d and 51-d. FIG. 4a-d is diagrammatic representation of the principles of the phase shift process and FIG. 5a-d is a diagrammatic illustration of a conventional process. FIGS. 4a and 5a are sectional views of reticles; FIGS. 4b and 5b show the amplitude of light on the reticles; FIGS. 4c and 5c depict the amplitude of light on wafers; and FIGS. 4d and 5d illustrate the intensity of light on the wafers. In FIG. 4a and 5a, reference numeral 1 stands for a substrate, 2 an opaque layer, 3 a phase shifter and 4 incident light.
In the conventional process, as illustrated in FIG. 5a, the substrate 1 formed as of quartz glass is provided thereon with the opaque layer 2 formed as of chromium, only to form a given pattern of light-transmitting regions. In phase shift lithography, however, the phase shifter 3 made up of a transparent film to cause phase reversal (with a phase difference of 180.degree.) is mounted on one of the adjacent light-transmitting regions on a reticle, as sketched in FIG. 4a. According to the conventional process, therefore, the amplitude of light on the reticle is in the same phase, as illustrated in FIG. 5b, as is the amplitude of light on the wafer, as depicted in FIG. 5c, with the result that the patterns on the wafer can no longer be separated from each other, as sketched in FIG. 5d. By contrast, the phase shift lithography enables the adjacent patterns to be distinctly separated from each other, as illustrated in FIG. 4d, because the light transmitting through the phase shifter is reversed in phase between the adjacent patterns, as depicted in FIG. 4b, so that the intensity of light on the pattern boundary can be reduced to zero. With the phase shift lithography, even patterns that cannot previously be separated from each other are thus made separable from each other, thereby achieving high resolution.
One example of conventional processes of producing phase shift reticles will now be explained with reference to FIG. 6a-m that is a series of sectional views illustrating the steps of producing a typical phase shift reticle. In FIG. 6a-m, reference numeral 11 denotes a quartz substrate, 12 a chromium film, 13 a resist layer, 14 ionizing radiations, 15 a resist pattern, 16 an etching gas plasma, 17 a chromium pattern, 18 an oxygen plasma, 19 a transparent film, 20 a resist layer, 21 ionizing radiations, 22 a resist pattern, 23 an etching gas plasma, 24 a phase shift pattern and 25 an oxygen plasma.
As illustrated in FIG. 6a, the chromium film 12 is first formed on the substrate 11 that is optically polished, and an ionizing radiation resist such as chrolomethylated polystyrene is uniformly coated thereon in conventional manners such as spin coating and heated for drying to form the resist layer 13 of about 0.1 to 2.0 .mu.m in thickness. The drying-by-heating treatment may usually be carried out at 80.degree. to 150.degree. C. for about 20 to 60 minutes, although varying depending on the type of resist used.
Then, as illustrated in FIG. 6b, a pattern is conventionally drawn on the resist layer 13 by the ionizing radiations 14 from photolithographic hardware such as an electron beam exposure system, then developed with a developer composed mainly of an organic solvent such as ethyl cellosolve or ester, and finally rinsed with an alcohol to form the resist pattern 15 such as one shown in FIG. 6c.
If required, heating and descumming treatments are further carried out to remove unnecessary resist portions such as resist scum and whiskers, if any, from the edge regions, etc., of the resist pattern 15. After that, as shown in FIG. 6d, the portions to be processed, that are exposed between the pattern (15) lines, i.e., the chromium layer 12 is etched dry by the etching gas plasma 16 to form the chromium pattern 17. As will be obvious to those skilled in the art, the formation of this chromium pattern 17 may be achieved as well, using wet etching in place of the dry etching with the etching gas plasma 16.
Following etching having been done in this manner, the resist pattern 15, i.e., the remaining resin is incinerated out by the oxygen plasma 18, as shown in FIG. 6e, thereby obtaining such a complete photomask as shown in FIG. 6f. It is noted that this incineration treatment using the oxygen plasma 18 may also be replaced by solvent removal.
Subsequently, this photomask is checked up to make some modification to the pattern, if required, followed by cleaning. After that, the transparent film 19 made as of SiO.sub.2 is formed on the chromium pattern 17, as shown in FIG. 6g. Then, as depicted in FIG. 6h, the ionizing radiation resist 20 such as chloromethylated polystyrene is formed on the transparent film 19 in similar manners as mentioned above, followed by alignment of the resist patterns 20, as shown in FIG. 6i. Subsequent drawing of a given pattern with the ionizing radiations 21, development and rinsing give the resist pattern 22, as illustrated in FIG. 6j.
Then, heating and descumming treatments are done, if required, and portions of the transparent film 19 exposed between the resist pattern (22) lines are then etched dry by the etching gas plasma 23 to form the phase shifter pattern 24, as illustrated in FIG. 6k. As will be obvious to those skilled in the art, the formation of this phase shifter pattern 24 may be achieved as well, using wet etching in place of the dry etching with the etching gas plasma 23.
Finally, the remaining resist is incinerated out by the oxygen plasma 25, as shown in FIG. 6i. Through the foregoing steps, such a phase shift mask containing the phase shifters 24 as shown in FIG. 6m is completed.
In the above-mentioned, conventional process of producing phase shift reticles, however, it is required to place the transparent film 19 to be formed with phase shifters under strict etching control in the depthwise direction. Especially because both the substrate 11 and the transparent film 19 are made of the same material based on SiO.sub.2, the substrate 11 is etched as well upon etching continued even after the etching of the transparent film 19 has been completed. This in turn makes the amount of phase shift of the phase shifters greater than 180.degree. and so renders precise pattern transfer difficult.
In view of the foregoing, the present applicant has come up with the provision of an etching stopper layer between the transparent film to be formed thereon with phase shifters and the substrate, thereby interrupting etching automatically (see JP-A-2-29801 and JP-A-2-181795). As set forth in these specifications, the etching stopper layer is made of such materials as tantalum, molybdenum, tungsten, silicon nitride and SnO.sub.2. Among these, SnO.sub.2 is currently used for etching stopper layers. SnO.sub.2 is a material that is well known to provide a transparent, electrically conductive film, but shows absorption in an ultraviolet wavelength range and so drops in terms of transmittance. In order to secure an i-line (of a mercury lamp light source (.lambda.=365 nm)) transmittance of 85% or higher, the film must be 15 nm or less in thickness. In addition, a dry etch ratio with respect to the phase shifter layer is insufficient or, in a more precise term, 10 or less.
Moreover, with an etching stopper layer of such materials it is not necessarily easy to interrupt etching precisely enough to be well very satisfactory.